Paper currency (usually benjamins) stored in a cowboy boot in the back of one's closet in attempt to avoid scrutiny from a significant other or spouse. This money is usually reserved for questionable activities that require heightened discretion.

setting: two guys walk into a strip club

Guy 1 (handed a wad of cash by Guy 2): No way I can accept five hundred dollars in cash
Guy 2: Don't worry about it, dude, this is boot money